1. Field of the Invention
The present invention relates to an ink jet printing apparatus that carries out printing by ejecting ink to a print medium. More specifically, the present invention relates to an ink jet printing apparatus that carries out preliminary ejection to recover the appropriate conditions of a print head as well as a preliminary ejecting method executed using this apparatus.
2. Description of the Related Art
Some printing apparatuses are used as means for printing images in a printer, a copier, a facsimile, or the like, or as print output equipment for composite electronic equipment, a workstation, or the like which includes a computer, a word processor, or the like. These printing apparatuses are configured to print images or the like on print media such as sheets or thin plastic sheets on the basis of image information (including all of output information such as text information). On the basis of their printing methods, these apparatuses can be classified into an ink jet type, a wire dot type, a thermal type, a laser beam type, and the like. Among them, an ink jet type printing apparatus (hereinafter referred to as an “ink jet printing apparatus”) carries out printing by ejecting from printing means including a print head, to a print medium. This method easily increases definition compared to the other printing methods. Further, this printing apparatus has various advantages: it operates fast and silently and is inexpensive. On the other hand, in recent years, color outputs such as color images have become more and more important. Accordingly, a large number of color ink jet printing apparatuses have been developed which provide high quality images equivalent to silver photographs.
In order to improve a printing speed, such an ink jet printing apparatus comprises a print head in which a plurality of print elements are integrally arranged and in which a plurality of ink ejection openings and liquid channels are integrated together. Furthermore, the apparatus is generally provided with a plurality of such print head in order to deal with color printing.
As shown in FIG. 1, the ink jet printing apparatus often uses a serial type printing method of printing the entire print medium by repeating a printing operation of executing printing while scanning a print head from which ink is ejected, along a guide rail and a paper feeding operation of feeding paper by a predetermined amount.
FIG. 2 is a schematic view showing an ejection opening surface of a print head. Ejection opening rows are formed in a direction perpendicular to a scanning direction of the print head. Further, the ejection opening rows for the respective ink colors are arranged parallel with the scanning direction of the print head.
In each of the ink ejection openings constituting the print head, if no ejecting operations are performed for a specified time, ink present close to the ejection opening may become more viscous or dust floating in the air may stick to the vicinity of the ejection opening. Consequently, ejection may be inappropriately carried out: the amount of ink ejected or the direction of ejection may become unstable during an ejecting operation. Thus, preliminary ejection, a kind of a recovery process, is periodically executed, after no ejecting operations are performed for the specified time, before an ejecting operation is started or during a printing operation. This enables the removal of the more viscous ink or the dust or droplets attached to the vicinity of the ejection opening, together with ejected ink.
With the above described serial type printing method, the print head moves to a preliminary ejection receiver provided in an area different from a print area. Then, ink is ejected to the preliminary ejection receiver the predetermined number of times at a predetermined ejection frequency. The preliminary ejection receiver is provided at, for example, a position opposite to a print head 102 at its home position.
After ejection, an ink droplet ejected from the ejection opening is often divided into a plurality of pieces before flying. The plurality of ink droplets obtained by the division include main droplets that are the largest ink droplets, satellites that are ink droplets smaller than the main droplets, and mists that are ink droplets finer than the satellites and flying at a low speed. This phenomenon of course occurs not only during a printing operation but also during a preliminary ejecting operation.
FIGS. 3A to 3C illustrate how an ejected ink droplet is divided. In these figures, reference numerals 301, 302, and 303 denote ink, just ejected ink, and meniscus. Reference numerals 304, 305, and 306 denote a main droplet, a satellite, and a floating mist.
As shown in FIG. 3A, ejection is started. Immediately after the start of the ejection, the ink is continuously ejected from a nozzle. Subsequently, in FIG. 3B, the meniscus 303, which results from the contraction of bubbles or the deformation of a piezoelectric element, retreats to move the ink 301 to the interior of the print head 102. The movement of the ink 301 causes the ejected ink 302 to be separated from the ink present inside the print head. This creates a speed distribution in the ejected ink 302. In FIG. 3C, the ink with the speed distribution is divided. This results in an ink droplet with the largest volume and the highest speed (main droplet 304), ink droplets having a smaller volume and a lower speed than the main droplet (satellites 305), and ink droplets having a much smaller volume and a much lower speed and floating in the air without reaching the preliminary ejection receiver (floating mists 306).
If each color nozzle row in the print head 102 undergoes preliminary ejection and when all of the nozzle rows are simultaneously subjected to preliminary ejection, power required for preliminary ejection may exceed the maximum power supplied to the ink jet printing apparatus. In this case, the ejection cannot be correctly executed. In view of such a problem associated with supplied power, the preliminary ejection is often executed a plurality of times for each color nozzle row. However, when the ejection openings or ejection opening rows in each color nozzle row are divided into groups for preliminary ejection, a time difference in preliminary ejecting operation occurs between the ejection openings or the ejection opening rows. The present inventors have found that this results in a color mixture problem.